SandpointsTheme/assets/css/postcss.config.js

const themeDir = __dirname + '/../../';

module.exports = {
    plugins: [
        require('postcss-import')({
            path: [themeDir]
            }),
      /* require('autoprefixer')({
       *     grid: true
       * }),
       * require('postcss-reporter'), */
    ]
}